LEDs emit an amount of light that depends upon the amount of current driven through them. Typical LED driver circuits regulate this current to keep it constant, assuming that if the driver current is constant, the LED light output will also be constant. In practice, however, the drive-current-to-light-output characteristic of LEDs may vary from LED to LED due to manufacturing defects or inconsistencies, different operating conditions for different LEDs, or other factors. These variations result in the light output of a particular LED at a given current being different from that of another LED of the same intended design. At most, existing systems may adjust LED light output in response to a sensed ambient light level, but this adjustment does not account or correct for variations in the LEDs themselves.
An example of an existing LED control circuit 100 appears in FIG. 1. One or more LEDs 102 are to be driven by a desired setpoint current ISP. A driver circuit, such as a pulse-width-modulation (“PWM”) circuit 104, emits a signal that has an average (DC) value that will result in the average current flowing through the LEDs being equal to the desired setpoint current ISP; via the action of, for example, high gain negative feedback; the output of the PWM may be filtered by a low-pass filter 106. The current driving the LEDs 102 is sampled (by, e.g., sensing the current with a resistor RS and amplifying the sensed current with an amplifier 108). A comparator 110 compares (e.g., subtracts) the sensed and amplified LED current with the desired current ISP and produces an error signal e. A dynamic compensator 112 generates a control signal u based on the received error signal e, and the PWM circuit 104 adjusts the duty cycle of its output pulses accordingly (e.g., it increases its duty cycle if the sensed LED current is lower than the desired current ISP or decreases its duty cycle if the sensed LED current is greater than the desired current ISP). The value of the desired current ISP may be varied (by, e.g., a dimmer circuit) to control the brightness of the light output by the LEDs 102.
The circuit 100 shown in FIG. 1 may be represented as a single-input-single-output (“SISO”) control loop 200, as shown in FIG. 2. The dynamic compensator 112 is represented by a first transfer function G(s,z), which (like the other transfer functions in FIG. 2) may be a continuous-time analog (s-domain) or a discrete-time digital (z-domain) function. A second transfer function P(s,z) represents both the PWM circuit 104 and the low-pass filter 106 in s or z domains, and a third transfer function KSRS represents the both the current sense resistor RS and the amplifier 108. A fourth transfer function H(s,z) represents the quasi-linear LED-current-to-light-output characteristics of the LEDs 102 over the range of regulated operating current. Because H(s,z) is not precisely known and varies from one set of LEDs to another, however, a given LED current value ILED produces different amounts of light produced by different LEDs. In other words, the actual value of the light output by the LEDs 102 at any time is determined by the static and/or dynamic characteristics of the transfer function H(s,z), which varies among LEDs intended to be identical.
A need therefore exists for a way to account and correct for variations in light output produced by variations in LED design, manufacture, operating conditions, or age.